The present invention relates to a casing structure for a steam turbine included in a thermal power generation system installed in, for example, a combined power plant, and a power generating system using the steam turbine provided with the casing structure.
Recently, many combined-cycle power plants provided with a gas turbine and a steam turbine in combination have been constructed. Generally, the improvement of steam conditions is directly related with the improvement of the efficiency of a power plant provided with a steam turbine. Therefore, the increase of the pressure and temperature of steam for driving a steam turbine included in a combined-cycle power generating system has been required to improve the efficiency of the power generating system and to enhance the output of the power generating system.
As shown in FIG. 8, a casing 110 of a high-pressure stage 5 of a conventional steam turbine for combined-cycle power generation is a single-wall casing. Usually, the thickness of the wall of the single-wall casing must be increased to improve the pressure withstand strength when inlet steam pressure is raised. In the event that the pressure and temperature of the steam are raised to improve the efficiency of the steam turbine provided with the conventional single-wall casing and to enhance the output of the same, an increased pressure stress and an increased thermal stress are induced in the casing owing to increase in the thickness of the wall of the casing. The casing is thus damaged by thermal fatigue or high temperature low-cycle fatigue during the operation, and the operation of the turbine affected.
The risk of steam leakage from the horizontal flange of the casing is increased by increase in thermal deformation of the casing, resulting in the marked degradation of the reliability of the steam turbine. Steam leakage involves the direct discharge of high-temperature, high-pressure steam into the atmosphere, which is fatal to the operation of the steam turbine, and increases the risk of fire and injury.
Since an excessively high thermal stress is induced in the casing having a thick wall at the start of the steam turbine, it must take a long time for starting up time of the turbine to reduce the level of the thermal stress. However, in a case, such as a combined-cycle power plant which is required quick start-up, the extension of the starting up time delays the start up of the combined-cycle power plant and increases the operating cost of the power generating system.
When the output of the steam turbine provided with a conventional single-wall casing structure is increased by raising the pressure and temperature of the main steam, the casing must be made of 12-Cr steel or 9-Cr steel, which has strength at high temperatures but expensive, instead of a conventional low alloy steel. The high material cost of the casing is a principal factor that increases the cost of the steam turbine.
The linear thermal expansion coefficients of the 12-Cr steel and the 9-Cr steel are smaller than those of conventional low alloy steels, typically CrMoV steels. Therefore, the thermal expansion of a casing made of 12-Cr steel or 9-Cr steel is smaller than that of the conventional casing. Thus, the expansion difference (the difference between the respective axial thermal expansions of the casing and the rotor with respect to a reference position corresponding to a thrust bearing of the turbine) is greater than that in the conventional turbine. This results in reduction of axial clearances between the rotor, i.e., a rotating body, and the components of the casing, i.e., stationary members. Due to this, the rotor and the components of the casing contact with each other, resulting in so-called axial-rubbing, causing the intense vibration of the shaft that hinders the continuation of the operation of the turbine.
Recently, a conventional combined-cycle steam turbine employs a double-wall casing structure including an inner casing 111 and an outer casing 112 entirely covering turbine stages from the high-pressure first stage 7 to the high-pressure exhaust stage 8 of the high-pressure section 5, as shown in FIG. 9, with a view to solving the foregoing problems. This known double-wall casing structure will be referred to as xe2x80x9ccomplete double-wall casing structurexe2x80x9d, for simplicity.
Basically, thermal stress induced in a casing is proportional to the temperature difference between the outer and inner surfaces of the casing. Supposing that a casing is a thin-wall cylindrical structure for simplicity, steady circumferential thermal stress due to the temperature difference between the outer and inner surfaces in the thin-wall cylindrical structure is expressed by: "sgr"xcex8t=0.714xcex1xc3x97Exc3x97T, where "sgr"xcex8t is steady thermal stress, xcex1 is the linear thermal expansion coefficient of the material of the thin-wall cylindrical structure, and T is the temperature difference between the outer and inner surfaces in the thin-wall cylindrical structure.
The temperature difference T1 between the outer and inner surfaces of the casing in the single-wall casing structure can be divided into 0.7xc3x97T1 in the outer casing of the double-wall casing structure, and 0.3xc3x97T1 in the inner casing of the same. Therefore, a steady thermal stress that will be induced in the inner casing of the double-wall casing structure is on the order of 0.7 times a thermal stress that will be induced in a single-wall casing structure. A steady thermal stress that will be induced in the outer casing of the double-wall casing structure is on the order of 0.3 times the thermal stress that will be induced in the single-wall casing structure. Thus, the steady thermal stress induced in the casing of the high-pressure section can be effectively reduced by using a double-wall casing structure.
Supposing that a casing is a thin-wall cylindrical structure for simplicity, circumferential stress induced in the thin-wall cylindrical structure due to the internal pressure therein is expressed by: "sgr"xcex8p=axc3x97p/t, where "sgr"xcex8p is circumferential stress, and t is the thickness of the thin-wall cylindrical structure. Thus, the pressure difference P1 between the internal and the external pressure of the casing of the single-wall casing structure can be divided into 0.7xc3x97P1 for the outer casing of a double-wall casing structure, and 0.3xc3x97T1 for the inner casing of the double-wall casing structure.
Supposing that a casing is a thin-wall cylindrical structure, the radius of an inner casing of a double-wall casing structure is about 0.9xc3x97a and that of an outer casing of the double-wall casing structure is about 1.5xc3x97a, where a is the radius of a single-wall casing. Therefore, the wall thickness of the single-wall casing is axc3x97P1/"sgr"1, the wall thickness of the outer casing of the double-wall casing structure is about 0.45xc3x97axc3x97P1/"sgr"2 and the wall thickness of the inner casing of the double-wall casing structure is about 0.63xc3x97axc3x97P1/"sgr"3, where "sgr"1 is a circumferential pressure stresses induced in the single-wall casing, and "sgr"2 and "sgr"3 are circumferential pressure stresses induced in the inner and outer casings of the double-wall casing structure, respectively.
If those circumferential stresses may be equal, i.e., "sgr"1="sgr"2="sgr"3, the respective wall thicknesses of the inner and outer casings of the double-wall casing structure may be about 0.63 times and about 0.45 times the wall thickness of the single-wall casing, respectively.
Conversely, the respective wall thicknesses of the inner and outer casings of the double-wall casing structure may be about 0.9 times and about 0.65 times the wall thickness of the single-wall casing, respectively, if it is desired to limit the pressure stress induced in the double-wall casing structure to a value 0.7 times the pressure stress induced in the single-wall casing. That is, the double-wall casing structure achieves reduction in the pressure stress while reducing the wall thickness.
Thus, the double-wall casing structure, as compared with the single-wall casing, is capable of reducing both steady thermal stress and pressure stress.
On the other hand, in a state such as the turbine is starting up, the temperature of the casing rises sharply, a high thermal stress is induced unsteadily in the casing and the casing is deformed at the same time. The respective magnitudes of the thermal stress and the thermal deformation are basically proportional to the temperature difference between the inner and outer surfaces of the casing. This temperature difference is greatly dependent on the wall thickness of the casing in a state where steam temperature and heat transfer coefficient change sharply, such as a state where the turbine is starting up.
The temperature difference between the inner and outer surfaces of the single-wall casing is large, because the inner surface of the single-wall casing is exposed directly to main steam and the outer surface of the same is exposed indirectly through lagging materials to the atmosphere. On the contrary, with the double-wall casing structure, the temperature differences between the inner and outer surfaces of the outer and inner casings are smaller by far than that in the single-wall casing. This is because, temperature difference between the inner and outer surface of the casing structure is distributed between the inner and outer casings, and temperature of steam applied to the inner and outer surfaces of the casing structure is distributed between the inner and outer casings.
Generally, the respective magnitudes of the thermal stress induced unsteadily in the casing and the thermal deformation of the casing are proportional to the temperature difference between the inner and outer surfaces of the casing. Therefore, the thermal stress induced unsteadily in the casings of the double-wall casing structure and the thermal deformation of the casings of the double-wall casing structure are smaller than those of the single-wall casing.
Steels for making the casing of a steam turbine have low thermal conductivities. Thus, if the casing has a thick wall, the conduction of heat from the inner surface to the outer surface of the casing takes a long time and the temperature difference between the inner and outer surfaces of the casing is large. In this respect, a double-wall casing structure, in which the respective wall thicknesses of the inner and outer casings may be smaller than that of a single-wall casing, is effective in suppressing an excessive increase of unsteady thermal stress and unsteady thermal deformation.
Since a double-wall casing structure, as compared with a single-wall casing, reduces the temperature difference between the internal and external atmospheres of the casing and the wall thickness, the temperature difference between the inner and outer surfaces of the casing can be greatly reduced. Consequently, an excessive increase of thermal stress and thermal deformation at the start of the turbine can be suppressed.
As mentioned above, a double-wall casing structure, as compared with a single-wall casing, is capable of reducing pressure stress, steady thermal stress, unsteady thermal stress and unsteady thermal deformation. Hence, double-wall casing structure is effective in preventing creep damage, thermal fatigue damage and damage resulting from high temperature low-cycle fatigue to the casing, and troubles, such as steam leakage through the horizontal flange of the casing.
However, the complete double-wall casing structure is inevitably costly. This is because, the outer casing of the complete double-wall casing structure included in the high-pressure section of a conventional large-capacity, industrial steam turbine and entirely covering a part of steam turbine from the high-pressure first stage 7 to the high-pressure exhaust stage 8 is very large. Since the complete double-wall casing structure has complicated construction and needs a large number of bolts for fastening a casing-horizontal-flange joining together an upper and lower halves of the casing, assembling and disassembling the turbine for periodic inspection or maintenance requires complicated work and a long time. Consequently, periodic inspection and the like need increase costs, periodic inspection needs a long time, whereby the availability of the power generating system decreases and power generating cost increases.
It is a still more important problem that the employment of the complete double-wall casing structure enhances the risk of axial-rubbing. The thermal expansion of the outer casing of the complete double-wall casing structure is small because the temperature of steam on the inner surface of the outer casing is approximately equal to the temperature of high-pressure exhaust steam, which is the lowest of those of steam in the high-pressure section.
Therefore, the axial elongation difference between a rotor shaft 10 which is a rotating member and a part of the casing which is a stationary member, in the vicinity of a shaft seal 9 on the high-pressure exhaust side, is very large, as compared with such an axial elongation difference in a single-wall casing structure. This results in reduction in axial clearance. Consequently, the complete double-wall casing structure comes into axial contact with the rotor shaft 10 to cause axial vibrations generally called rubbing vibrations. Excessively intense axial vibrations hinder the operation of the turbine and increase greatly the risk of significantly getting the reliability of the turbine worse.
If the axial clearance is increased to reduce such risk, the amount of steam leakage through the shaft seal increases to make the performance of the turbine worse, which is undesirable from the viewpoint of performance. Actually, a considerably large axial clearance, as compared with an axial clearance required by the single-wall casing, must be secured in the shaft seal of the complete double-wall casing structure. Consequently, the leakage of steam through the shaft seal increases and make the performance of the turbine worse.
Those problems are true of other industrial steam turbines that are required to operate on high-pressure, high-temperature steam as well as of steam turbines for combined-cycle power generation.
Furthermore, since the steam turbine for combined-cycle power generation is a small-capacity or a medium-capacity, the flow of main steam is low and blade height is liable to become short and the performance of the steam turbine is worse. Therefore, by evaluating the relation between the root circle diameter and tip circle diameter of the moving blades desirable in respect of structural strength and performance of a steam turbine for combined-cycle power generation to enable the steam turbine to exercise satisfactory performance, the decline of the performance must be prevented.
The present invention has been made in view of the foregoing circumstances and it is therefore an object of the present invention to solve problems of securing strength at elevated temperatures and of preventing steam leakage that arise when high-pressure, high-temperature steam is used for driving a steam turbine, and problems of preventing the occurrence of rubbing due to excessive elongation difference and of minimizing steam leakage from shaft seals.
With the foregoing object in view, the present invention provides an axial-flow steam turbine, which includes a high-pressure section provided with a turbine casing, the turbine casing having: a double-wall casing structure, having an inner casing and an outer casing, arranged at an area corresponding to stages from a high-pressure first stage to a predetermined high-pressure stage arranged on an upstream side of a high-pressure final stage; and a single-wall casing structure arranged at an area corresponding to stages from a stage located next to said predetermined high-pressure stage to said high-pressure final stage.
The partial double-wall casing structure is preferably applied to a steam turbine that employs main steam having a pressure not lower than 120 kgf/cm2 and a temperature not lower than 550xc2x0 C., and has a rated output power of 120 MW or above.
It is also preferable that the double-wall casing structure is arranged so that steam pressure in a steam passage corresponding to the double-wall casing structure is 90 kgf/cm2 or above, or that steam temperature in a steam passage corresponding to the double-wall casing structure is 480xc2x0 C. or above.
The present invention also provides an axial-flow steam turbine, which includes a high-pressure section and an intermediate-pressure section, wherein steam discharged from the high-pressure section is reheated by a steam reheater, and the steam thus reheated is supplied to the intermediate-pressure section, wherein said high-pressure section has a turbine casing having: a double-wall casing structure, having an inner casing and an outer casing, arranged at an area corresponding to stages from a high-pressure first stage to a predetermined high-pressure stage arranged on an upstream side of a high-pressure final stage; and a single-wall casing structure arranged at an area corresponding to stages from a stage located next to said predetermined high-pressure stage to said high-pressure final stage, wherein said intermediate-pressure section has a turbine casing having: a double-wall casing structure, having an inner casing and an outer casing, arranged at an area corresponding to stages from an intermediate-pressure first stage to a predetermined intermediate-pressure stage arranged on an upstream side of an intermediate-pressure final stage; and a single-wall casing structure arranged at an area corresponding to stages from an intermediate-pressure stage located next to said predetermined intermediate-pressure stage to said intermediate-pressure final stage, and wherein said inner casings of the said high-pressure section and intermediate-pressure section are integrated.
The partial double-wall casing structures of the high and intermediate pressure sections are preferably applied to a steam turbine that employs main steam having a pressure not lower than 120 kgf/cm2 and a temperature not lower than 550xc2x0 C., and has a rated output power of 120 MW or above, and wherein a temperature of reheat steam is 550xc2x0 C. or above.
It is also preferable that the double-wall casing structures of the high and intermediate pressure sections are arranged so that steam temperature in a steam passage corresponding to the double-wall casing structure is 480xc2x0 C. or above.
In the event that the aforementioned partial double-wall casing structure is applied, it is preferable that the outer casing is made of a low alloy steel containing 1 to 3% Cr, such as a CrMoV alloy steel, and the inner casing is made of a Cr steel containing 8 to 10% Cr or a Cr steel containing 9.5 to 12.5% Cr. Alternatively, both the outer and inner casings may be made of a low alloy steel containing 1 to 3% Cr, such as CrMoV steel.
It is preferable that, in the stages of the high-pressure section corresponding to the double-wall casing structure, 0.85 less than Dr/Dt less than 0.95, where Dr is root circle diameter including roots of moving blades and Dt is tip circle diameter including tips of the moving blades.
The steam turbine provided with the partial double-wall casing structures is suitable for use in combined-cycle power generating systems, thermal power plants without being combined with a gas turbine, or industrial power generating systems.